In data transmission systems, each terminal is provided with a local clock source for the timing of operations at that terminal. All transmission links, both terrestrial and satellite, vary in electrical length with time and, therefore, in propagation delay. This variation in propagation delay causes a change in the phase of the transmitted data bits with respect to the receiver clock. Viewed in another way, there is a transient change in the rate of the incoming data bits. An elastic store in the receiving termal compensates for this variation. The elastic store is a buffer which introduces a variable delay to the incoming data bits to synchronize the data with the clock in the receiver terminal. The elastic store receives the data at the varying incoming rate and supplies it to other receiver circuitry at the receiver clock rate.
Fixed capacity buffers with independent read/write cycles have been used in data transmission networks to compensate for propagation delay variations. Such buffers, hereinafter referred to as commutator-type stores, are described in section 26.2 of "Transmission Systems for Communications" published by Bell Telephone Laboratories, revised Fourth Edition, December 1971, pages 616, 617. While commutator-type stores can provide continuously variable delay, their implementation becomes quite costly if more than a few bits of storage capacity is needed.
Variable capacity stores, such as variable length shift registers, can store a large number of data bits economically. The delay through such stores can be adjusted by varying the number of cells used for storing data. However, as each cell provides a fixed amount of delay, a shift register store can only provide incremental delay changes that only approximate the actual delay required. Such approximation can cause errors in signal processing. Consequently, a large capacity elastic store capable of providing continuously variable delay is desirable.